Morphology and structure of polymer layers protecting dental enamel against erosion.

OBJECTIVES: Human dental erosion caused by acids is a major factor for tooth decay. Adding polymers to acidic soft drinks is one important approach to reduce human dental erosion caused by acids. The aim of this study was to investigate the thickness and the structure of polymer layers adsorbed in vitro on human dental enamel from polymer modified citric acid solutions. METHODS: The polymers propylene glycol alginate (PGA), highly esterified pectin (HP) and gum arabic (GA) were used to prepare polymer modified citric acids solutions (PMCAS, pH 3.3). With these PMCAS, enamel samples were treated for 30, 60 and 120s respectively to deposit polymer layers on the enamel surface. Profilometer scratches on the enamel surface were used to estimate the thickness of the polymer layers via atomic force microscopy (AFM). The composition of the deposited polymer layers was investigated with X-ray photoelectron spectroscopy (XPS). In addition the polymer-enamel interaction was investigated with zeta-potential measurements and scanning electron microscopy (SEM). RESULTS: It has been shown that the profilometer scratch depth on the enamel with deposited polymers was in the range of 10nm (30s treatment time) up to 25nm (120s treatment time). Compared to this, the unmodified CAS-treated surface showed a greater scratch depth: from nearly 30nm (30s treatment time) up to 60nm (120s treatment time). Based on XPS measurements, scanning electron microscopy (SEM) and zeta-potential measurements, a model was hypothesized which describes the layer deposited on the enamel surface as consisting of two opposing gradients of polymer molecules and hydroxyapatite (HA) particles. SIGNIFICANCE: In this study, the structure and composition of polymer layers deposited on in vitro dental enamel during treatment with polymer modified citric acid solutions were investigated. Observations are consistent with a layer consisting of two opposing gradients of hydroxyapatite particles and polymer molecules. This leads to reduced erosive effects of citric acid solutions on dental enamel surfaces.

Templating α-helical poly(L-lysine)/polyanion complexes by nanostructured uniaxially oriented ultrathin polyethylene films.

We report a templating effect of uniaxially oriented melt-drawn polyethylene (MD-PE) films on α-helical poly(L-lysine)/poly(styrenesulfonate) (α-PLL/PSS) complexes deposited by the layer-by-layer (LBL) method. The melt-drawing process induced an MD-PE fiber texture consisting of nanoscale lamellar crystals embedded in amorphous regions on the MD-PE film surface whereby the common crystallographic c axis is the PE molecular chain direction parallel to the uniaxial melt-drawing direction. The MD-PE film and the α-PLL/PSS deposit were analyzed by atomic force microscopy (AFM) and in situ attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) using polarized light as a complementary method. Both methods revealed that α-PLL/PSS complexes adsorbed at the MD-PE surface were anisotropic and preferentially oriented perpendicular to the crystallographic c direction of the MD-PE film. Quantitatively, from AFM image analysis and ATR-FTIR dichroism of the amide II band of the α-PLL, mean cone opening angles of 12-18° for both rodlike α-PLL and the anisotropic α-PLL/PSS complexes with respect to the PE lamellae width direction were obtained. A model for the preferred alignment of α-PLL along the protruding PE lamellae is discussed, which is based on possible hydrophobic driving forces for the minimization of surface free energy at molecular and supermolecular topographic steps of the PE surface followed by electrostatic interactions between the interconnecting PSS and the α-PLL during layer-by-layer adsorption. This study elucidates the requirements and mechanisms involved in orienting biomolecules and may open up a path for designing templates to induce directed protein adsorption and cell growth by oriented polypeptide- or protein-modified PE surfaces.

Pectin, alginate and gum arabic polymers reduce citric acid erosion effects on human enamel.

OBJECTIVES: Consumption of acidic soft drinks may lead to the dissolution and softening of human enamel, known as erosion. The first aim of this in vitro study was to test the hypothesis that food-approved polymers added to citric acid solutions (CAS) decrease the erosion of human dental enamel compared to citric acid solutions without these polymers. The second aim was to test the hypothesis that these polymers added to CAS form a polymer layer directly on the eroded enamel surface. METHODS: Enamel samples were obtained by embedding pieces of non-erupted human third molars in resin, grinding, and polishing them. CAS with pH values (pH: 2.3, 3.3 and 4.0) of typical soft drinks were prepared and modified by adding one of the following food-approved polymers (1%, w/w): highly esterified pectin (HP), propylene glycol alginate (PGA) and gum arabic (GA). The enamel samples were exposed to these polymer-modified citric acid solutions (PMCAS) or CAS not containing polymers, respectively, for different time periods (30, 60 and 120s). Atomic force microscopy (AFM)-based nanoindentation was used to analyze the nanomechanical properties of the treated enamel samples and the control samples. The enamel nanohardness and the reduced elastic modulus of the samples treated with PMCAS were statistically analyzed (ANOVA, t-test) and compared to the mechanical properties of the samples treated with unmodified CAS. Thus treated enamel samples were imaged by scanning electron microscopy (SEM) to investigate the surface morphology of the different enamel samples. RESULTS: Enamel samples treated with PMCAS containing GA or PGA showed statistically significantly higher nanohardness (p<0.05) compared to samples treated with CAS. PMCAS containing HP did not reduce the enamel nanohardness loss significantly compared to the CAS treated enamel samples. The enamel samples eroded with PMCAS show generally a smoother surface compared to the enamel surfaces of samples treated only with CAS as detected by SEM. Therefore, it is hypothesized that the polymers possibly adsorb on the eroded enamel surface. SIGNIFICANCE: The present in vitro erosion study shows that some of the polymers used in this study may possibly adsorb like a protective layer directly onto the human enamel surface. For GA and PGA this possibly formed polymer layer reduces the erosive effects of citric acid solutions as shown by nanoindentation measurements.

A novel two-level microstructured poly(N-isopropylacrylamide) hydrogel for controlled release.

The aim of this work was to demonstrate that conventional poly(N-isopropylacrylamide) (PNIPAAm) hydrogels can improve their shrinkage and release properties solely due to the introduction of a heterogeneous density fluctuation-based microstructure. To this end, a novel structurally engineered PNIPAAm hydrogel was designed and compared with a chemically similar, but homogeneous, PNIPAAm hydrogel reference. For the two-step preparation PNIPAAm microgels were firstly synthesized with surface amine groups and further functionalized with polymerizable acrylate groups. In the second step the microgels, themselves acting as crosslinkers, were crosslinked to form a bulk network by inter-connecting the microgels with linear PNIPAAm chains. Although the chemical composition of the newly prepared hydrogel was generally the same as conventional PNIPAAm hydrogels (a relative control), significantly improved shrinkage properties and a more efficient "on-off" switching induced by temperature modulations were observed for the novel gel as compared with the homogeneous reference. These improved shrinkage properties were ascribed to the novel structure, which is believed to enable rapid shrinking of the small microgel crosslinkers and, thereupon, the generation of a sufficient number of diffusion channels for quick water release. Rhodamine B and ibuprofen (IBU) as model compounds were completely released from this novel gel at 20 degrees C, whereas at temperatures above the lower critical solution temperature release stopped after initial 40% and 70% "bursts" for rhodamine B and IBU, respectively, due to shrinkage of the gel network. This approach may provide an avenue to design temperature-sensitive drug delivery systems with state of the art switching properties and fast release kinetics by combining the here presented innovative strategy with complementary enhancements, such as the introduction of porosity.